1. Field of the Invention
The present invention relates to an apparatus and method for chemical sensing, and more specifically to gas detection sensors using heterostructures that form a two-dimensional electron gas channel.
2. Technical Background
Chemical sensing, such as gas detection, is required in a number of different applications such as in situ air pollution monitoring and industrial manufacturing. In many applications it is important that the sensors exhibit at least some of the following properties: being operable at room temperature; being relatively low cost; being suitable for use in a battery operated apparatus, i.e. having a low power consumption; being reliable and not subject to degradation; and having a fast response with a high sensitivity and resolution.
Ion sensitive FETs and gas FETs are known types of semiconductor based sensor that use a FET configuration with a conductive channel that responds to changes in the ion/gas concentration. In one arrangement changes in the ion/gas concentration result in a change in the potential formed between the gate and the channel. In gate-less configurations the sensors respond to charges or dipoles formed at the channel surface, by channel depletion or enhancement.
Recently, gas sensors based on heterostructures that form a high mobility two dimensional electron gas (2DEG) have been proposed. The 2DEG that forms in such structures provides a highly conductive channel that can be operated similar to the conventional FET based sensors, but with the advantage of a much higher electron mobility which can result in a higher sensitivity.
Hung et al. “Study of a New Field-Effect Resistive Hydrogen Sensor Based on a Pd/Oxide/AlGaAs Transistor”, IEEE Transactions on Electron Devices, Vol. 52, No. 5, pp 1224-1231, May 2007 describes a hydrogen sensor with a Pd gate electrode on an oxide layer. Hydrogen dissociates at the surface of the Pd gate electrode and forms a dipole layer. The presence of a dipole layer causes a significant decrease in channel resistance in the 2DEG formed in an InGaAs layer. Such a structure can only be used however for sensing of a limited set of gases that can dissociate at the surface of certain metals.
MEMS resonator structures using 2DEGs have also been proposed as gas sensors. Patent application US2009/0174014 teaches a MEMS structure with a 2DEG that can be used as an actuator but which can also be used as a resonator coated with adsorbing or absorbing coating. Binding of the chemical species to the coating changes the resonance properties of the structure, for example due to a mass change of the resonator. Such a resonance structure can work with any suitable coating but requires the sensor to be constantly electrically actuated in use, which results in relatively high power consumption.
In certain aspects, the present invention aims to provide an improved apparatus and method for chemical sensing.